METALLURGY. 265 



fon have removed every doubt I entertained on this head. With 

 regard to ferrugineous stones, in which the calx of iron is not 

 much dephlojsticated, this decomposition is easily un<!ertood ; 

 for this calx irndually bero'nes more dephlogisticated by the action 

 ot the water and air, attracts water and fixed air. and loses its 

 adherence with the siliceous, or other stony parti. 1 s : this is seen 

 to happen to basaltc-s, toadstone, ferragfaMOM 1 mestone, c. In 

 other Atones, this decomposition may arise from their containing 

 cali-art ous earth in a caustic state, or manganese ; for these will 

 gradually attract water and hxed air, and then swell, burst, and 

 loosen the whole texture of the stone, as we see happen to bricks 

 that contain lime. Thus also t>lass is decomposed by long expo, 

 sure to the air, the alkali attracting water and aerial acid. Mor. 

 tar, on the contrary, hardens by long exposure to the air, because, 

 though >he aenal acid be attracted, yet a great part of the water 

 exi.ales*." The changes produced by the long expo-ure of bodies 

 to ihe air, and the causes of them, deserve a more minute investi- 

 gation than has hitherto been bestowed on them ; some advantage 

 nii^ht, perh:) ps, be derived from the inquiry to our manufactu- 

 rers ; for 1 have cause to think that iron, which has been exposed 

 to the air tor three or four years, is a very different substance from 

 the same iron when just made : and the same observation will pro. 

 bably hold with respect to copper and brass. But to return from 

 this digression. The calamine of Bohemia contains iron ; most of 

 our English calamine contains lead ; and there are some sorts 

 which contain both iron and lead, and other metals in different 

 proportions : these sorts can seldom be freed from the extraneous 

 metals ; and hence, in the ordinary method of making brass, they 

 Mill be mixed with it, being fusible in the degree of heat usually 

 employed in making brass. Cramer mentions a very ingenious 

 method of making brass, by which, if it should be thought neces. 

 sary to do it, the brass may be preserved pure from these hetero. 

 geneous mixtures. He orders the calamine and charcoal to be mix- 

 ed with moistened clay, and rammed to the bottom of the melting 

 pot, and the copper mixed with charcoal to be placed upon the 

 clay ; then, the proper degree of heat being applied, the vapour 

 of the zinc contained in the calamine will ascend tnn>u,li (he clay, 

 and attach itself to the copper, but the iron or lead contained m the 



* Elements of Mio. by K. Kir wan. p. 111. 



